What is the C# static fields naming convention?

The Microsoft guidelines are silent about private fields, they are only concerned with publicly visible members.

Common conventions are camelCase, _camelCase and even sometimes the hangover from C++/MFC m_camelCase.

If you use camelCase without a prefix, your property backing fields will differ from the property name only in case, which is not a problem in C#, but won't work in a case-insensitive language like VB.NET.

So many people, including myself, like to use an underscore prefix so that the same standards can be used in all languages. In my experience, underscore is much more common than m_.

According to MSDN, use Pascal Case for static fields. I always chuckle when MSDN and StyleCop contradict each other :).

So if you are following MSDN standards, the correct way is:

private static string MyString;

According to StyleCop (and with the default settings), the correct way to name most fields (as specified below) is with a lowercase letter at the start.

SA1306: FieldNamesMustBeginWithLowerCaseLetter

... Field and variable names must begin with a lower-case letter, unless the field is public or internal, const, or non-private and readonly. In these cases, the field should begin with an upper-case letter.

See also SA1309: FieldNamesMustNotBeginWithUnderscore.